1. Field of the Invention
The present invention relates to the aqueous processing of the surfaces of large, flat or nearly flat objects, and more specifically, it relates to the use of the Marangoni effect in processing such substrates.
2. Description of Related Art
Cleaning the surfaces of large parts is a critical processing step in many industrial operations. Also, developing of photoresist and wet etching are essential steps during processing of large flat optics, silicon wafers and flat panel display substrates. Rinsing and drying are essential follow-up steps. Aqueous-based agents are necessary for some of these steps and are desirable overall for cleaning due to the problems associated with flammable and hazardous solvents. Drying of water films remaining from the rinse step is problematic without resorting to heat and turbulent gas flow, and these drying processes can leave behind residues.
The cleaning of objects in critical applications is often accomplished by ultrasonic energy or megasonic energy incident on the surface of the object contacted by a liquid medium. An example of an ultrasonic cleaning device is disclosed in U.S. Pat No. 4,788,992. An example of a megasonic cleaning device is disclosed in U.S. Pat. No. 5,339,842. The former invention utilizes an air knife to dry parts after the cleaning operation, while the latter invention does not discuss the drying operation. Both processes would benefit by utilizing Marangoni drying as put forth in this application. The drying of semiconductor wafers and flat panel substrates is often accomplished by centrifugation. This method becomes unwieldy for large substrates and is known to generate particles which can contaminate the surface being dried.
Another class of drying apparatus for semiconductor substrates is known as vapor dryers. Examples of such systems are U.S. Pat. Nos. 5,371,950, 5,351,419, 5,243,768 and 5,183,067. In these units, vapor from a solvent such as isopropanol or a fluorinated liquid is introduced in the vicinity of a substrate and condenses on the substrate, flushing the original liquid on the substrate. The condensate is then caused to flash evaporate from the part, leaving it dry. Such processing units require large amounts of organic and/or flourinated solvents, heating and cooling modules, and elaborate solvent recycling equipment. They are not conducive to in-situ cleaning and drying of an object as part of an integrated process unit. A Marangoni drying process has been described (Leenaars, et al.. Langmuir, 6, 1701, 1990) in which a nitrogen gas flow containing solvent vapors is blown onto a part which is being withdrawn from a quiescent reservoir of water. This apparatus demonstrates the principle of Marangoni drying, but is limited in practice by drying nonuniformities on large-area surfaces due to nonuniformity of vapor impingement on the surface, and by lack of water flow near the three-phase contact line to maintain a strong, one-dimensional surface tension gradient. The prior art methods have been nominally effective; however, there is still a need for improvements in the drying of the substrates. These improvements are needed to reduce processing steps and production costs, and to provide uniform drying and a higher degree of cleanliness during the drying operation.